244 ,^ 



lines, Instead of continuing downward. If, on the other hand, p is greater 

 than Pj', the pressure on the plate will rise suddenly to the value p^ at 

 the instant tg and will then remain constant, as Illustrated by the upper of 

 the two broken lines in Figure 10. 



The distributions of pressure in the water, as plotted in Figure 11, 

 will be modified in ways to correspond. The part of the reflected wave that 

 is reflected from the water surface after the occurrence of cavitation will 

 be modified so as to contain higher pressures, since the pressure at the wa- 

 ter surface is higher than it would have been if the water had continued in 

 contact with the plate. In Figure 11, on each of the later curves there will 

 be a point representing the instantaneous position of that part of the re- 

 flected wave v;hich was reflected Just as cavitation began; such a point is 

 Indicated by a on Curve E. The pressure to the right of this point contains 

 a component that was reflected from the free water surface instead of from 

 the plate and hence will lie somewhat higher than it would in the absence of 

 cavitation, as is suggested in Figure 11 by the broken line ab. 



As an alternative, cavitation might begin in the water itself. In 

 such a case the analysis given in foregoing sections becomes applicable. 

 Cavitation will start at a definite position as well as at a definite time. 

 It might begin, for example, at Q in Figure 11; this point would then repre- 

 sent the position of that plane in the water, parallel to the plate, at which 

 the pressure first sinks to the breaking-pressure p^. 



From this initial plane, a plane breaking-front will advance a 

 short distance toward the plate, while another one will follow the reflected 

 wave toward the left, moving a little more rapidly than this wave so as al- 

 ways to be in the position at which the total pressure equals p^. Successive 

 positions of the latter breaking-front are indicated in Figure 11 by Q', Q", 

 Q "' . Behind this front, or on the right in the figure, lies the cavitated 

 region, in which the pressure equals the cavity pressure p^ . The boundary of 

 this region on the side toward the plate is not shown in Figure 11, since its 

 position can only be inferred from a more detailed study of the motion of the 

 water near the plate. The uniform pressure p^ behind the breaking- front, on 

 the assumption that p^ is greater than p^, is illustrated for a certain in- 

 stant of time by the broken line behind Q "' . Thus, the part of Curve F to 

 the left of Q'", up to 12 feet from the plate, represents the part of the re- 

 flected wave that got past Q before cavitation began, diminished somewhat 

 through being partially overtaken by the breaking-front which moves at first 

 at supersonic velocity. The remainder of Curve F is replaced by the uniform 

 pressure in the cavitated region or near the plate by an undetermined modi- 

 fied pressure. 



